The present invention can be more easily understood in terms of a simple exemplary system. Consider a telephone conversation in which a person talks into a microphone whose output is digitized and then transmitted to a second person via various telephone lines and switch systems. The speaker at the second person's location receives a sequence of digital values that are then played back to the second person. In general, the received sequence will differ from the transmitted sequence because of errors introduced by the transmission system, digital-to-analog converters, and analog to digital converters. For example, noise in the transmission system results in some of the digital values in the transmitted sequence being altered. One goal of a denoising system is to remove as many of these noise errors as possible.
The simple example discussed above is an example of a more general problem that is encountered in a wide range of applications. In general, an input digital signal that consists of a sequence of “symbols” is transmitted through a “communication link” and is received as an output digital signal at the output of the communication link. The output digital signal also consists of a sequence of “symbols”. Each of the symbols is chosen from a predetermined set of symbols, referred to as an alphabet. The output signal is assumed to be written in the same alphabet as the input signal.
In the simplest case, the signals are binary signals in which the alphabet consists of the symbols “0” and “1”. In this case the input and output signals consist of a sequence of 0s and 1s. However, other alphabets are commonly used. For example, a digitized signal in which each symbol is represented by an integer between 0 and M−1 is commonly used in broadband data transmission systems for connecting users to the Internet via a digital subscriber loop (DSL).
While the above examples refer to communication systems, it should be noted that this type of noise problem is present in a number of data processing systems. For example, the storage of data files on a magnetic disk drive can be viewed as the transmission of a digital signal through a communication link, the disk drive. The input signal is a sequence of symbols, e.g., bytes of data, which are chosen from a predetermined alphabet. In the case of byte data, each symbol has an integer value chosen from the set [0, 1, . . . , 255]. The retrieved file from the disk drive also consists of a sequence of symbols chosen from this set. The input signal symbols are processed by the electronics of the disk drive and stored in the form of localized magnetic fields that are read to generate the output signal. Noise in the digital to analog circuitry that converts the symbols to and from the magnetic fields introduces errors into the output signal. In addition, the magnetic fields can be altered during storage by random events that introduce additional errors.
Similarly, digital photography may be viewed as involving the transmission of a signal through a channel that corrupts the signal. In this case, the signal is the image, which is corrupted by noise in the photodetectors.